Method of casting solid propellant rocket motors



June 29, 1965 L. T. JAGIELLO METHOD OF CASTING SOLID PROPELLANT ROCKET MOTORS Filed Aug. 50, 1965 VIIIIIIIIZI/(Il/IA METAL FUEL-OXIDIZER BINDER CURING AGENT PROPELLANT ROCKET MOTOR MOTOR MOTOR FORMULATING CASTING SPINNING CURING STEP STEP FIG. 3.

STEP STEP FINISHED ROCKET MOTOR INVENTOR. LEONARD T. JAGIELLO ATTORNEY.

United States Patent 3,192,289 METHOD OF CASTING SOLID PROPELLANT ROCKET MOTORS Leonard T. Jagiello, China Lake, Califi, assignor to the United States of America as represented by the Secretar-y of the Navy Filed Aug. 30, 1963, Ser. No. 305,888 3 Claims. (Cl. 2643) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States without the payment of any royalties thereon or therefor.

The present invention relates to a method of casting solid propellant rocket motors.

Since shortly after World War II, the emphasis in solid propellant research and development has gradually switched from extruded double base propellants to the cost composite propellant-s. 'By definition, the composite propellants are those in which the fuel and the oxidizers are discrete, separate chemical substances in close physical contact with each other. Unlike the double base propellants which are homogeneous (except for small quantities of additives), the composite propellants are distinctly heterogeneous, consisting essentially of one or more solid phases suspended in a rubbery binder or matrix. Most of the present day composite propellants use powdered aluminum or other metals as a fuel, and ammonium perchlorate, potassium percholarate or ammonium nitrate as the oxidizer. The size limitations of extruded grains, imposed by the processing equipment available, rules out certain types of propellants, including conventional double base, for large missiles. Some composites are extruded and the use of those suffers from the size limitations. Some of these grains have been cemented together for large motors. Grains of much larger size and more complicated shapes can be obtained by casting or sometimes by molding. In the conventional cast propellant grain the constituents are more or less uniformly distributed. In instances where high energy particle additives are mixed with the other propellant constituents (such as some of the metals which are usually of higher density than the other propellant constituents) the particles adjacent the ignition surface or burning surface have been known to create some undesirable characteristics such as difiicult ignition, rough initiation of burning and formation of combustion products which are highly erosive. The present invention contemplates casting a solid propellant mixture in place in the motor and while the mixture is in a viscous state, spinning the motor to generally cause a migration of the denser, high energy particles of the propellant mixture away from the internal ignition surface, thereby forming a solid propellant grain which will have improved ignitability, smoother initiation of burning, and cleaner combustion products which are initially less erosive and may aid in preventing flameouts in aircraft.

An object of the present invention is the provision of a method for casting solid propellant grains which will have improved ignition and burning charatceristics.

Another object is to provide a casting technique which is applicable to various propellant compositions.

A further object of the invention is the provision of a casting technique which will result in improved bonding of the propellant to the rocket motor casing.

Still another object is to provide a method of forming cast solid propellant motors with certain desirable prescribed burning and/ or thrust charatceristics by providing a means of varying the composite constituents in certain portions of the grain during the casting process.

Other objects and many of the attendant advantages 3,192,289 Patented June 29, 1965 ice of this invention will be appreciated as the same become better understood by reference to the following detailed description and the accompanying drawing in which:

FIG. 1 is an elevational view in partial section showing a finished rocket motor manufactured according to the present invention;

FIG. 2 is a cross-sectional view of the rocket motor taken on line 2-2 of FIG. 1; and

FIG. 3 is a block diagram showing the main steps of the method for manufacturing a rocket motor according to this invention.

Referring to FIG. 1 there is shown the rocket motor 10 in its final form wherein the solid propellant grain 12 having an internal perforation 13 is bonded directly to metal casing 11. Grain 12 is shown with the portion near the internal perforation 13 having a light concentration of metal particles which get denser toward the outer edges of the grain away from the internal ignition surface. FIG. 2 is a cross-section of the finished rocket motor shown in FIG. 1 showing propellant grain 12 with the dense metal particles in varying degrees of concentration throughout the grain, but having the greatest degree of concentration at the outer edge bonded to casing 11 and lesser' concentration toward internal perforation 13.

In FIG. 3 the main steps by which the solid propellant rocket motor is manufactured are shown in a flow chart which is described in detail below.

The example hereinafter set forth best illustrates the present method for mixing and casting a rocket motor:

The oxidizer (ammonium perchlorate) crystals were ground to a predetermined particle size, ranging around 200 microns and density of about .04 lb./in. under controlled atmosphere and blended with a metal fuel (such as aluminum) of about 30p. size and density of about .06 lb./in. The binder-fuel (polybutadiene acrylic acid) and the oxidizer-metal blend now mixed together along with the epoxy resin in a sigma-blade mixer to form a thick dough, which was subjected to vacuum to remove air bubbles. The dough or mix was now poured or pumped directly into a rocket motor casing around a central mandrel. This material was poured at a temperature of about F. and the temperature was raised to about 176 F. While the dough or mix was still viscous the motor casing assembly was spun about the long axis of the motor at predetermined rate and for a duration sufficient to cause the denser metal particles of the propellant composition due to centrifugal force to migrate to the outer edges of said grain and away from the internal ignition surface of the grain. The motor assembly was cured for about 48 hours at 176 F. then the mandrel was removed. The temperature was raised to 180 F. and curing continued for another 10 hours. The motor was now allowed to cool to room temperature. The resultant grain structure was of a lower metal particle concentration near the internal perforation.

It is postulated that a variety of structural characteristics of a cast solid propellant could result from the above described improved process or method which would greatly enhance ignition, burning, thrust and specific impulse of a rocket motor. Varying the axis, magnitude and duration of spin and/or varying the formulations by including mixture of high energy, high density fuels, etc., could result in a new type solid propellant with improved specific performance. Propellants could be cast in conamazes '2 a centric sections with diiferent mixtures and additives in each section.

This invention utilizes inertial forces due to spin and/ or compound motion during the casting process to impart a variety of characteristics to a solid propellant rocket motor using a more or less uniform mix of propellant constituents and additives.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. lit is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An improved process for casting composite propellant grains which comprises (a) blending an inorganic oxidizer with a metal powder to form a dry oxidizer-metal mixture;

(b) mixing said dry mixture into a binder-fuel and curing agent until a mixture forms;

(c) pouring said viscous mixture at an elevated temperature into a rocket motor having a centrally positioned mandrel;

(d) spinning said motor about its long axis until a substantial part of the metal in said mixture migrates away from the center of the motor due to centrifugal force;

(e) curing said motor for a predetermined time at an elevated temperature.

2. in a method for casting composite propellant grains in a rocket motor having a centrally positioned mandrel which comprises (a) blending oxidizer crystals up to 290,41, in size and metallic particles up to 30a in size with a binder and curing agent until a thick, viscous mixture results,

(13) casting said mixture into said motor around said mandrel; the improvement which comprises (1) spinning said motor along its long axis at a predetermined rate for a predetermined length of time so that a substantial amount of the metal 41 particles in said mixture migrate away from that area of the mixture surrounding the mandrel; (2) curing said motor at an elevated temperature for several hours until the grain sets; (3) removing said mandrel from said motor and (4) curing at ambient temperature for several days.

3. In a method for casting a composite propellant composition in a rocket motor having a centrally positioned mandrel which comprises (a) stirring together the following constituents until a homogeneous viscous mixture results 7 Percent by weight 1) Polybutadiene acrylic acid 11.06

(2) Epoxy resin 2.94 (3) Ammonium perchlorate 70.0 (4) Aluminum 16.0

(b) casting said mixture into said motor around said mandrel, the improvement which comprises (1) spinning said motor along its long axis so that the metal particles in said mixture migrate away from the center of the motor resulting in a lower metal particle concentration near the mandrel; (2) curing until a solid grain forms within the motor.

References Qited by the Examiner UNITED STATES PATENTS 1,896,973 2/33 Rutenberg 264-311 2,248,098 7/41 Cornelius et al 117-95 2,773,287 12/56 Stout. 2,887,728 5/59 Usab 18-583 3,017,670 1/62 Zweig 78-59 3,049,043 8/62 Milani ct al. 264-3 CARL D. QUARFORTH, Primary Examiner. ALEXANDER H. BRODMERKEL, Examiner. 

1. AN IMPROVED PROCESS FOR CASTING COMPOSITE PROPELLANT GRAINS WHICH COMPRISES (A) BLENDING AN INORGANIC OXIDIZER WITH A METAL POWDER TO FORM A DRY OXIDIZER-METAL MIXTURE; (B) MIXING SAID DRY MIXTURE INTO A BINDER-FUEL AND CURING AGENT UNTIL A MIXTURE FORMS; (C) POURING SAID VISCOUS MIXTURE AT AN ELEVATED TEMPERATURE INTO A ROCKET MOTOR HAVING A CENTRALLY POSITIONED MANDREL; (D) SPINNING SAID MOTOR ABOUT ITS LONG AXIS UNTIL A SUBSTANTIAL PART OF THE METAL IN SAID MIXTURE MIGRATES AWAY FROM THE CENTER OF THE MOTOR DUE TO CENTRIFUGAL FORCE; (E) CURING SAID MOTOR FOR A PREDETERMINED TIME AT AN ELEVATED TEMPERATURE. 